12-0-tetradecanoyl-phorbol-13-acetate (TPA), a potent tumor promoter in vivo, can modulate the proliferative response of mitogen-activated lymphocytes in vitro. TPA itself is nonmitogenic, but it can enhance DNA synthesis by acting as an analogue of interleukin 1, a macrophage lymphokine. In cultures of macrophage-depleted lymphocytes, TPA synergizes with concanavalin A (Con A) to allow a full proliferative response. Additionally, TPA can synergize with wheat germ agglutinin, a nonmitogenic lectin, to cause mitogenesis. TPA and the calcium ionophore, A23187, together replace the requirement for lectins. The two compounds, TPA and A23187, may activate protein kinases ordinarily activated through lectin-receptor interactions. Assays of protein kinase activity in a nondenaturing gel system have revealed several different protein kinases, at least one of which is stimulated by treating the cells with Con A and one of which is depressed by treatment with TPA. TPA treatment of cells for some hours prior to the lectin also depresses the response to the lectin. In this case, a change in T-cell regulation, possibly T suppressor cells may occur. By marking T cells with sheep red blood cell (SRBC) rosettes, we have identified four subpopulations. TPA, but not the nonpromoting phorbol compounds, can change the distribution of rosetting cells. Specifically, a decrease in T cells rosetting with neuraminidase-treated SRBC (EN+) correlates with an increase in inhibition of lectin stimulation. Attempts are being made to isolate these cells and to test them directly for suppressor activity. (N)